Use memoization get _*_path() functions

This shouldn't make a big difference in the CLI/extension
usage, but might improve the performances of long-running
instances, or people misusing the API.

libmat2/exiftool.py

@@ -1,3 +1,4 @@
+import functools
 import json
 import logging
 import os
@@ -56,6 +57,7 @@ class ExiftoolParser(abstract.AbstractParser):
             return False
         return True
 
+@functools.lru_cache()
 def _get_exiftool_path() -> str:  # pragma: no cover
     possible_pathes = {
         '/usr/bin/exiftool',              # debian/fedora

libmat2/video.py

@@ -1,3 +1,4 @@
+import functools
 import os
 import logging
 
@@ -130,6 +131,7 @@ class MP4Parser(AbstractFFmpegParser):
     }
 
 
+@functools.lru_cache()
 def _get_ffmpeg_path() -> str:  # pragma: no cover
     ffmpeg_path = '/usr/bin/ffmpeg'
     if os.path.isfile(ffmpeg_path):